1. Field of the Invention
The invention relates to an improved method for establishing the coverage area of a Wide-Area Differential Global Positioning System (WADGPS), and specifically to predict the level of accuracy when navigating within a coverage area to transmit information and determine a user's position.
2. Description of the PriorArt
WADGPS have been developed for a multiple of applications, including aircraft navigation. Such systems use a combination of the U.S. Department of Defense Global Positioning System (GPS) Space Vehicles (SV), a network of Ground Reference Stations (GRS), a Central Control Station (CCS), and differential corrections continuously broadcast from either a ground or a space based reference point. A ground transmitter may be placed at a fixed geographic location, while a space based transponder may be placed aboard a geosynchronous earth orbiting SV (GEO SV). The CCS generates the differential corrections using the GRS observable measurements and the differential correction transmitter continuously broadcasts the WADGPS information to users within the wide area. One such system being developed ior the U.S. Federal Aviation Administration is the Wide-Area Augmentation System (WAAS), as described in specification FAA-E-2892.
Various methods have been proposed in the prior art to assess the effectiveness of such GPS augmentation systems. The standard GPS coverage models quantify accuracy and availability in terms of Dilution of Precision. While generally effective, these prior art methods do not capture the spatial and temporal dynamics of a WADGPS system. The additional GRSs within a WADGPS enables the space vehicle Signal-in-Space (SIS) errors to be decomposed into their ephemeris and clock components. The present invention utilizes such a decomposition to more closely approximate the SV SIS accuracy and project these error sources onto a lattice of user locations to predict WADGPS navigation accuracy. The inventive method allows prediction of the effective user position accuracy over a wide geographical area. This will determine the operational characteristics of an existing WADGPS system or optimize a proposed WADGPS system configuration.
Two alternatives exist to evaluate user position determination accuracy in a WADGPS. The first alternative involves the use of a discrete simulation of the WADGPS network and the evaluation of the various performance metrics by enumeration. However, such a discrete simulation requires a precise definition of the network and numerous Monte Carlo scenarios to generate meaningful statistics. A second alternative is the use of an analytical model that captures the underlying physics and dynamics of the WADGPS configuration. The present invention uses such analytical models to predict the WADGPS navigation accuracy and its effective coverage area.